It is commonplace in the electronic art to construct devices in and/or on thin wafer substrates. Common wafer materials are semiconductors such as for example, silicon, germanium and gallium-arsenide, and dielectrics such as for example, sapphire, alumina, quartz, doped garnets, combinations of dielectrics and semiconductors, and other materials.
Wafers of these materials are often prepared in large boules which are then sliced into raw wafers typically 50-200 mm in diameter and 0.025-0.25 mm thick. Before they can be used, the raw wafers must be ground or lapped to the final desired thickness and polished and etched to remove sawing and other surface damage.
Lapping and polishing is conventionally carried out in large orbital lapping machines well known in the art. The wafers to be lapped and/or polished are typically placed in circular openings in a lap plate whose thickness is about equal the desired final wafer thickness. The wafers and lap plate are placed between rotating platens of the lap machine. A slurry containing an appropriate polishing media, lubricant and (sometimes) a chemical etchant is introduced between the wafers and the lapping pads on the platens. As the platens of the lap machine turn, gear rings engage the outer perimeter of the lap plate so that they rotate about their centers at the same time that they revolve around the central axis of the lap machine. Thus, the wafers move in an orbital fashion with respect to the polishing pads on the platens. This is desirable for achieving wafers whose thickness is as uniform as possible. Such equipment and methods are well known in the art.
One of the difficulties encountered with prior art lapping and polishing methods is imperfect wafer flatness and planarity. Some of the wafers polished or lapped using such prior art methods show small deviations from perfect flatness and thickness uniformity. The wafers may have a slight thickness taper from one edge to another, or a slight crown toward the center, or be slightly dish-shaped. All such variations are undesirable. Such deviations may arise during the lapping or polishing process or may arise from non-uniformities in the sawing and/or grinding operations that precede lapping or polishing. In either case, these non-uniformities are undesirable since they make it much more difficult to achieve uniform characteristics in the electronic devices constructed in or on the wafers.
It has been discovered that the flatness, uniformity and surface perfection of the wafers obtained using such prior art lapping or polishing methods is very dependent on how the lapping or polishing slurry is distributed on the lap pads. It has also been discovered that prior art means and methods result in a non-uniform distribution of slurry. Further, prior art means and methods for distributing the slurry are not well adapted to correcting wafer non-uniformities that may be present in the raw wafers before the lapping or polishing operation.